1. Field of the Invention
The present invention relates to an antenna for wireless communication for use in a portable terminal device such as a cellular telephone or PDA (personal digital assistant) or in a PC (personal computer) or CF (compact flash) card provided with communication capability. The present invention relates particularly to an inverted-F plate antenna. The present invention relates also to a wireless communication device incorporating such an inverted-F plate antenna.
2. Description of the Prior Art
In wireless communication devices such as cellular telephones, to miniaturize the devices, it is customary to mount an antenna on a printed circuit board on which circuits for achieving various functions are mounted. One type of antenna that is mounted on a printed circuit board is an inverted-F plate antenna, which has the advantage of occupying a small volume. FIG. 17 shows a typical shape of such an inverted-F plate antenna.
As shown in FIG. 17, on a printed circuit board 7 having a ground pattern 8h formed over its top surface, an inverted-F plate antenna 1 having an elongate conductor portion 1a is mounted. This inverted-F plate antenna 1 is formed by cutting a piece of metal having an appropriate shape out of sheet metal and then bending it into the desired shape so as to have a grounding conductor portion 1b and a feeding conductor portion 1d. The grounding conductor portion 1b has a mounting conductor portion 1c formed at the free end thereof and connected to a ground pattern 8h, and the feeding conductor portion 1d has a mounting conductor portion 1e formed at the free end thereof and connected to a feeding point 9. In this way, the grounding conductor portion 1b and the feeding conductor portion 1d are formed perpendicular to the elongate conductor portion 1a, which is arranged parallel to the printed circuit board 7, thereby forming an inverted-F shape.
Moreover, between the inverted-F plate antenna 1 thus mounted and the printed circuit board 7, a non-metallic spacer 6 is inserted to keep the inverted-F plate antenna 1 securely in position and to determine the gap between the inverted-F plate antenna 1 and the printed circuit board 7. This spacer 6 is inserted between the elongate conductor portion 1a of the inverted-F plate antenna 1 and the printed circuit board 7, and is located away from the grounding conductor portion 1b and the feeding conductor portion 1d. 
Many antennas have a main plane of polarization, and exhibit a lower gain for radio waves polarized perpendicularly to the plane of polarization (in the present specification, a xe2x80x9cgainxe2x80x9d denotes not the gain in a single direction but the average gain in all directions). For example, a dipole antenna as shown in FIG. 18 exhibits directivity patterns as shown in FIG. 19. Specifically, this antenna exhibits a significant gain mainly for vertically polarized radio waves as indicated by a solid line and a low gain for horizontally polarized radio waves as indicated by a broken line. That is, theoretically, a perfect linear dipole antenna exhibits no gain for horizontally polarized radio waves. On the other hand, a monopole antenna having infinite expanses of ground as shown in FIG. 20 exhibits a radiating pattern only above the ground surface, and exhibits no directivity pattern below the ground surface. It is to be noted that the direction parallel to the printed circuit board 7 will be referred to as horizontal.
The inverted-F plate antenna 1 shown in FIG. 17 exhibits directivity patterns similar to those of the monopole antenna shown in FIG. 20, and thus exhibits a low gain on the side of the printed circuit board 7 facing away from the antenna 1 and for horizontally polarized radio waves polarized parallel to the printed circuit board 7. For this reason, the inverted-F plate antenna 1 shown in FIG. 17 or a similar antenna radiates radio waves with comparatively low signal strength in the direction parallel to the printed circuit board 7. Accordingly, in a wireless communication device incorporating such an inverted-F plate antenna 1, so long as the device is supposed to be used in a particular position, the inverted-F plate antenna 1 is so arranged as to exhibit a gain in the desired direction when the device is placed in that particular position.
However, when an antenna is incorporated in a CF card provided with wireless communication capability and the CF card is used while being inserted into a PDA, or when an antenna is incorporated in a PDA provided with wireless communication capability and this PDA is used, as shown in FIG. 21, the PDA may be used in any position, for example, while being held vertically or horizontally or placed on a desk. In FIG. 21, reference numeral 150 represents a PDA, and reference numeral 151 represents a base station. Ideally, an antenna like this incorporated in a CF card or PDA should be able to transmit and receive both vertically and horizontally polarized radio waves, and its directivity patterns should not be lopsided in a particular direction but be spherical, i.e., uniform in all directions.
These aims can be achieved, for example, with a diversity system, i.e., by providing two antennas, one having the main plane of polarization in the horizontal direction and the other having the main plane of polarization in the vertical direction, and choosing whichever is offering better reception at every moment. However, where miniaturization is crucial as in a CF card, an antenna incorporated therein needs to be as compact and simple as possible, and therefore a diversity system, which requires two antennas, is unsuitable.
An object of the present invention is to provide an inverted-F plate antenna that exhibits a uniform gain in all directions and that exhibits less lopsided directivity patterns than ever, and to provide a wireless communication device incorporating such an inverted-F plate antenna.
To achieve the above object, according to one aspect of the present invention, an inverted-F plate antenna is provided with: an elongate conductor portion provided so as to face a printed circuit board; a grounding conductor portion provided at one side of the elongate conductor portion and electrically connected to a ground pattern formed on the printed circuit board; a feeding conductor portion provided at one side of the elongate conductor portion and electrically connected to a feeding point formed on the printed circuit board. Here, the ground pattern formed on the printed circuit board has a portion thereof removed in part of the area on the printed circuit board facing the elongate conductor portion.
According to another aspect of the present invention, a wireless communication device is provided with: an inverted-F plate antenna for at least either transmitting a communication signal to outside or receiving a communication signal from outside. The inverted-F plate antenna is provided with: an elongate conductor portion provided so as to face a printed circuit board; a grounding conductor portion provided at one side of the elongate conductor portion and electrically connected to a ground pattern formed on the printed circuit board; a feeding conductor portion provided at one side of the elongate conductor portion and electrically connected to a feeding point formed on the printed circuit board. Here, the ground pattern formed on the printed circuit board has a portion thereof removed in part of an area on the printed circuit board facing the elongate conductor portion.